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Date: Thu, 3 Mar 2022 10:32:33 +0800
From: Chao Yu <chao@...nel.org>
To: Jaegeuk Kim <jaegeuk@...nel.org>
Cc: Jing Xia <jing.xia@...soc.com>,
linux-f2fs-devel@...ts.sourceforge.net,
Zhiguo Niu <zhiguo.niu@...soc.com>,
linux-kernel@...r.kernel.org
Subject: Re: [f2fs-dev] [PATCH] f2fs: fix to avoid potential deadlock
On 2022/3/3 3:45, Jaegeuk Kim wrote:
> On 03/02, Chao Yu wrote:
>> On 2022/3/2 13:26, Jaegeuk Kim wrote:
>>> On 03/02, Chao Yu wrote:
>>>> ping,
>>>>
>>>> On 2022/2/25 11:02, Chao Yu wrote:
>>>>> On 2022/2/3 22:57, Chao Yu wrote:
>>>>>> On 2022/2/3 9:51, Jaegeuk Kim wrote:
>>>>>>> On 01/29, Chao Yu wrote:
>>>>>>>> On 2022/1/29 8:37, Jaegeuk Kim wrote:
>>>>>>>>> On 01/28, Chao Yu wrote:
>>>>>>>>>> On 2022/1/28 5:59, Jaegeuk Kim wrote:
>>>>>>>>>>> On 01/27, Chao Yu wrote:
>>>>>>>>>>>> Quoted from Jing Xia's report, there is a potential deadlock may happen
>>>>>>>>>>>> between kworker and checkpoint as below:
>>>>>>>>>>>>
>>>>>>>>>>>> [T:writeback] [T:checkpoint]
>>>>>>>>>>>> - wb_writeback
>>>>>>>>>>>> - blk_start_plug
>>>>>>>>>>>> bio contains NodeA was plugged in writeback threads
>>>>>>>>>>>
>>>>>>>>>>> I'm still trying to understand more precisely. So, how is it possible to
>>>>>>>>>>> have bio having node write in this current context?
>>>>>>>>>>
>>>>>>>>>> IMO, after above blk_start_plug(), it may plug some inode's node page in kworker
>>>>>>>>>> during writebacking node_inode's data page (which should be node page)?
>>>>>>>>>
>>>>>>>>> Wasn't that added into a different task->plug?
>>>>>>>>
>>>>>>>> I'm not sure I've got your concern correctly...
>>>>>>>>
>>>>>>>> Do you mean NodeA and other IOs from do_writepages() were plugged in
>>>>>>>> different local plug variables?
>>>>>>>
>>>>>>> I think so.
>>>>>>
>>>>>> I guess block plug helper says it doesn't allow to use nested plug, so there
>>>>>> is only one plug in kworker thread?
>>>
>>> Is there only one kworker thread that flushes node and inode pages?
>>
>> IIRC, =one kworker per block device?
>
> If there's one kworker only, f2fs_write_node_pages() should have flushed its
> plug?
No, f2fs_write_node_pages() failed to attach local plug into current->plug due to
current has attached plug from wb_writeback(), and also, f2fs_write_node_pages()
will fail to flush current->plug due to its local plug doesn't match current->plug.
void blk_start_plug_nr_ios()
{
if (tsk->plug)
return;
...
}
void blk_finish_plug(struct blk_plug *plug)
{
if (plug == current->plug) {
__blk_flush_plug(plug, false);
current->plug = NULL;
}
}
Thanks,
>
>>
>> Thanks,
>>
>>>
>>>>>>
>>>>>> void blk_start_plug_nr_ios(struct blk_plug *plug, unsigned short nr_ios)
>>>>>> {
>>>>>> struct task_struct *tsk = current;
>>>>>>
>>>>>> /*
>>>>>> * If this is a nested plug, don't actually assign it.
>>>>>> */
>>>>>> if (tsk->plug)
>>>>>> return;
>>>>>> ...
>>>>>> }
>>>>>
>>>>> Any further comments?
>>>>>
>>>>> Thanks,
>>>>>
>>>>>>
>>>>>> Thanks,
>>>>>>
>>>>>>>
>>>>>>>>
>>>>>>>> Thanks,
>>>>>>>>
>>>>>>>>>
>>>>>>>>>>
>>>>>>>>>> Thanks,
>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>>> - do_writepages -- sync write inodeB, inc wb_sync_req[DATA]
>>>>>>>>>>>> - f2fs_write_data_pages
>>>>>>>>>>>> - f2fs_write_single_data_page -- write last dirty page
>>>>>>>>>>>> - f2fs_do_write_data_page
>>>>>>>>>>>> - set_page_writeback -- clear page dirty flag and
>>>>>>>>>>>> PAGECACHE_TAG_DIRTY tag in radix tree
>>>>>>>>>>>> - f2fs_outplace_write_data
>>>>>>>>>>>> - f2fs_update_data_blkaddr
>>>>>>>>>>>> - f2fs_wait_on_page_writeback -- wait NodeA to writeback here
>>>>>>>>>>>> - inode_dec_dirty_pages
>>>>>>>>>>>> - writeback_sb_inodes
>>>>>>>>>>>> - writeback_single_inode
>>>>>>>>>>>> - do_writepages
>>>>>>>>>>>> - f2fs_write_data_pages -- skip writepages due to wb_sync_req[DATA]
>>>>>>>>>>>> - wbc->pages_skipped += get_dirty_pages() -- PAGECACHE_TAG_DIRTY is not set but get_dirty_pages() returns one
>>>>>>>>>>>> - requeue_inode -- requeue inode to wb->b_dirty queue due to non-zero.pages_skipped
>>>>>>>>>>>> - blk_finish_plug
>>>>>>>>>>>>
>>>>>>>>>>>> Let's try to avoid deadlock condition by forcing unplugging previous bio via
>>>>>>>>>>>> blk_finish_plug(current->plug) once we'v skipped writeback in writepages()
>>>>>>>>>>>> due to valid sbi->wb_sync_req[DATA/NODE].
>>>>>>>>>>>>
>>>>>>>>>>>> Fixes: 687de7f1010c ("f2fs: avoid IO split due to mixed WB_SYNC_ALL and WB_SYNC_NONE")
>>>>>>>>>>>> Signed-off-by: Zhiguo Niu <zhiguo.niu@...soc.com>
>>>>>>>>>>>> Signed-off-by: Jing Xia <jing.xia@...soc.com>
>>>>>>>>>>>> Signed-off-by: Chao Yu <chao@...nel.org>
>>>>>>>>>>>> ---
>>>>>>>>>>>> fs/f2fs/data.c | 6 +++++-
>>>>>>>>>>>> fs/f2fs/node.c | 6 +++++-
>>>>>>>>>>>> 2 files changed, 10 insertions(+), 2 deletions(-)
>>>>>>>>>>>>
>>>>>>>>>>>> diff --git a/fs/f2fs/data.c b/fs/f2fs/data.c
>>>>>>>>>>>> index 76d6fe7b0c8f..932a4c81acaf 100644
>>>>>>>>>>>> --- a/fs/f2fs/data.c
>>>>>>>>>>>> +++ b/fs/f2fs/data.c
>>>>>>>>>>>> @@ -3174,8 +3174,12 @@ static int __f2fs_write_data_pages(struct address_space *mapping,
>>>>>>>>>>>> /* to avoid spliting IOs due to mixed WB_SYNC_ALL and WB_SYNC_NONE */
>>>>>>>>>>>> if (wbc->sync_mode == WB_SYNC_ALL)
>>>>>>>>>>>> atomic_inc(&sbi->wb_sync_req[DATA]);
>>>>>>>>>>>> - else if (atomic_read(&sbi->wb_sync_req[DATA]))
>>>>>>>>>>>> + else if (atomic_read(&sbi->wb_sync_req[DATA])) {
>>>>>>>>>>>> + /* to avoid potential deadlock */
>>>>>>>>>>>> + if (current->plug)
>>>>>>>>>>>> + blk_finish_plug(current->plug);
>>>>>>>>>>>> goto skip_write;
>>>>>>>>>>>> + }
>>>>>>>>>>>> if (__should_serialize_io(inode, wbc)) {
>>>>>>>>>>>> mutex_lock(&sbi->writepages);
>>>>>>>>>>>> diff --git a/fs/f2fs/node.c b/fs/f2fs/node.c
>>>>>>>>>>>> index 556fcd8457f3..69c6bcaf5aae 100644
>>>>>>>>>>>> --- a/fs/f2fs/node.c
>>>>>>>>>>>> +++ b/fs/f2fs/node.c
>>>>>>>>>>>> @@ -2106,8 +2106,12 @@ static int f2fs_write_node_pages(struct address_space *mapping,
>>>>>>>>>>>> if (wbc->sync_mode == WB_SYNC_ALL)
>>>>>>>>>>>> atomic_inc(&sbi->wb_sync_req[NODE]);
>>>>>>>>>>>> - else if (atomic_read(&sbi->wb_sync_req[NODE]))
>>>>>>>>>>>> + else if (atomic_read(&sbi->wb_sync_req[NODE])) {
>>>>>>>>>>>> + /* to avoid potential deadlock */
>>>>>>>>>>>> + if (current->plug)
>>>>>>>>>>>> + blk_finish_plug(current->plug);
>>>>>>>>>>>> goto skip_write;
>>>>>>>>>>>> + }
>>>>>>>>>>>> trace_f2fs_writepages(mapping->host, wbc, NODE);
>>>>>>>>>>>> --
>>>>>>>>>>>> 2.32.0
>>>>>>
>>>>>>
>>>>>> _______________________________________________
>>>>>> Linux-f2fs-devel mailing list
>>>>>> Linux-f2fs-devel@...ts.sourceforge.net
>>>>>> https://lists.sourceforge.net/lists/listinfo/linux-f2fs-devel
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> Linux-f2fs-devel mailing list
>>>>> Linux-f2fs-devel@...ts.sourceforge.net
>>>>> https://lists.sourceforge.net/lists/listinfo/linux-f2fs-devel
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